1. Field of the Invention
The present invention relates to a transmission rate control method for controlling transmission rate in uplink, a mobile station, and a radio base station.
2. Description of the Related Art
In a conventional mobile communication system, in an uplink from a mobile station UE to a radio base station Node B, a radio network controller RNC is configured to determine a transmission rate of a dedicated channel, in consideration of radio resources of the radio base station Node B, an interference volume in an uplink, transmission power of the mobile station UE, transmission processing performance of the mobile station UE, a transmission rate required for an upper application, and the like, and to notify the determined transmission rate of the dedicated channel by a message of a layer-3 (Radio Resource Control Layer) to both of the mobile station UE and the radio base station Node B.
Here, the radio network controller RNC is provided at an upper level of the radio base station Node B, and is an apparatus configured to control the radio base station Node B and the mobile station UE.
In general, data communications often cause burst traffic compared with voice communications or TV communications. Therefore, it is preferable that a transmission rate of a channel used for the data communications is changed fast.
However, as shown in FIG. 1, the radio network controller RNC integrally controls a plurality of radio base stations Node B in general. Therefore, in the conventional mobile communication system, there has been a problem that it is difficult to perform fast control for changing of the transmission rate of channel (for example, per approximately 1 through 100 ms), due to processing load, processing delay, or the like.
In addition, in the conventional radio network controller RNC, there has been also a problem that costs for implementing an apparatus and for operating a network are substantially increased even if the fast control for changing of the transmission rate of the channel can be performed.
Therefore, in the conventional mobile communication system, control for changing of the transmission rate of the channel is generally performed on the order from a few hundred ms to a few seconds.
Accordingly, in the conventional mobile communication system, when burst data transmission is performed as shown in FIG. 2A, the data are transmitted by accepting low-speed, high-delay, and low-transmission efficiency as shown in FIG. 2B, or, as shown in FIG. 2C, by reserving radio resources for high-speed communications to accept that radio bandwidth resources in an unoccupied state and hardware resources in the radio base station Node B are wasted.
It should be noted that both of the above-described radio bandwidth resources and hardware resources are applied to the vertical radio resources in FIGS. 2B and 2C.
Therefore, the 3rd Generation Partnership Project (3GPP) and the 3rd Generation Partnership Project 2 (3SGPP2), which are international standardization organizations of the third generation mobile communication system, have discussed a method for controlling radio resources at high speed in a layer-1 and a media access control (MAC) sub-layer (a layer-2) between the radio base station Node B and the mobile station UE, so as to utilize the radio resources effectively. Such discussions or discussed functions will be hereinafter referred to as “Enhanced Uplink (EUL)”.
In the field of Enhanced Uplink (EUL), as shown in FIG. 3, a transmission rate of uplink (to be exact, a transmission power ratio between an “Enhanced Dedicated Physical Data Channel (E-DPDCH)” and a “Dedicated physical Control Channel (DPCCH)”) is increased, when a “Relative rate Grant Channel (RGCH)” which is received from non-serving cells includes “UP” command. The “UP” command is a command which orders the increase of the transmission rate of uplink (Refer to a Non-Patent literature 1: 3GPP TSG-RAN TS25.309 v6.2.0).
When the data to be transmitted in a transmission buffer of the mobile station UE become empty, the radio base station Node B transmits a “DTX” using the RGCH, so as to maintain the transmission rate of uplink in the mobile station UE. In other words, the radio base station Node B does not transmit anything, so as to maintain the transmission rate of uplink in the mobile station UE.
However, in a conventional mobile communication system using the Enhanced Uplink, the mobile station UE may detect a receive signal in the RGCH as “+1” and determine that the mobile station UE receives the “UP” command from the radio base station Node B, even when the radio base station Node B transmits the “DTX”.
When determining that the mobile station UE receives the “UP” command from the radio base station Node B, the mobile station UE is configured to increase the transmission rate by a predetermined value from the previously used transmission rate (the previously used transmission power ratio).
Thus, when the mobile station UE does not transmit any data in the previous TTI (transmission time interval), the mobile station UE is configured to increase the transmission rate by the predetermined value from 0 kbps. As a result, the mobile station UE cannot maintain the transmission rate.
Accordingly, there has been a problem that the radio base station Node B has to perform the additional processing, for example, instructing the transmission rate to the mobile station UE using an “Absolute rate Grant Channel (AGCH)”, which causes a transmission delay.